The generation of a reference gas having a known concentration of a volatile liquid substance is very useful, for instance, in the art of gas calibration, olfactometry and medicine.
Gas calibrators are widely used by laboratories and industries for the calibrations of sensors or other analytical equipments. Such gas calibrators usually use a permeation device, such as the one described in U.S. Pat. No. 4,399,942 (Chand) issued on Aug. 23, 1983.
Olfactometers are used for helping panelists to compare and evaluate odor intensities. In fact, odors are difficult to classify and to measure with automatic devices because most odors are issued from complex chemical compositions. The nuisance of an odor often results from a synergetic effect between odor pollutants and the only practical method for globally evaluating the odor nuisance is the use of a human test panel along with an olfactometer.
For decades, the sensory technique for measuring odors was the threshold method. This method typically involves the sampling of the odor pollutants into hermetical bags and the laboratory testing thereof by submitting the samples to dilution with odorless air until at least half of the members of a panel cease to perceive the odor pollutants in the diluted sample. The relative intensity of the odor pollutants are then deducted from the numbers of dilutions of the original samples. The number of dilutions is the figure used as a reference for quantifying the nuisance.
Because the threshold of the human nose is an extreme of sensitivity, it varies from a person to another and also for the same person subjected to different environmental conditions. Therefore, the threshold technique is not very accurate.
To achieve better results, a large number of panelists is required through the use of statistical methods, thus requiring considerable time and resources to perform a threshold analysis of a single mixture. One alternative way for evaluating the odor nuisance is to provide air samples and compare them with a reference gas carrying a known concentration of a reference substance with a clearly experienceable specific odor. In this technique, known as suprathreshold odor referencing, dilutions of the reference gas are achieved until both odors are believed to be equivalent by a panelist. The concentration of the reference gas is then recorded for comparing with the results of the other panelists. The practices for referencing the odor intensity in the suprathreshold region are well described in standard ASTM E544-75, which was reapproved in 1988.
Like the olfactometer described in the standard ASTM E544-75, the olfactometers found in the prior art are generally made from standard laboratory parts. The parts often contain glass and are heavy, fragile and energy consuming. These early suprathreshold designs were either using bottled reference gases, or were generating their own reference gas by passing an odorless gaseous medium over the surface of volatile liquid substance or by bubbling the odorless gaseous medium into the liquid. An example of such olfactometer is disclosed in Sweeten J. et al., Journal of the Air Pollution Control Association, Volume 34, No. 12, pp 1208-1213, December 1984. The construction of an olfactometer controlling a reference gas in an accurate and reliable manner from standard components was requiring performance compromises in lieu of complex and bulky devices. In spite of all efforts, these devices generally require analytical apparatuses for monitoring their proper operation.
More recent olfactometers have reference gas generating systems which are immersed into a liquid bath with a controlled temperature for minimizing the effect of the temperature of the gaseous medium on the partial pressure of the volatile liquid substance. An example of such device is disclosed in U.S. Pat. No. 4,934,386 (Walker et al.) issued on Jun. 19, 1990. However, the olfactometers with a liquid bath are not suitable for fully portable olfactometers.
Until now, the reference odor was provided to the panelists through a plurality of sniffing ports having respective concentrations and continuous flow of the reference odor, or through a single sniffing port where the reference odor is generally diluted by a factor of 2. A reason for this is the lack of reliable reference gas generators in which the concentration of the vapors of the volatile reference substance is known and maintained. It is not relevant to try to obtain precise dilution factors if the reference gas cannot be controlled adequately. Things become worse for portable equipment, as many of the standard laboratory apparatus cannot be incorporated into transportable equipment, be it glass parts, liquid baths, or calibration and analytical instruments that were teamed with most of the previous art olfactometers. The present invention is believed to resolve that problem.